This invention relates to heat-curable compositions comprising epoxide resins, certain Lewis acid latent hardeners for epoxide resins, and organic oxidising agents, and to the use of these compositions as laminating and moulding resins, surface coatings, adhesives, and the like.
Latent hardeners for epoxide resins (i.e., substances containing, on average, more than one 1,2-epoxide group per molecule) are very well known. Such materials may be mixed with the epoxide resin and stored at room temperature or below for extended periods without causing cross-linking of the resin: only when the mixture is heated does crosslinking take place to any significant extent. Typical such hardeners are Lewis acids, such as complexes of boron trifluoride, especially the amine complexes. The use of such complexes is described by Lee and Neville in "Handbook of Epoxy Resins", McGraw-Hill Inc., 1967, Chapter 11, pages 2 to 8, and by May and Tanaka in "Epoxy Resins--Chemistry and Technology", Marcel Dekker Inc., 1973, p. 202.
These complexes are usually employed as hardeners for epoxide resins wherein at least one of the 1,2-epoxide groups is present as a 2,3-epoxypropyl group, typically diglycidyl ethers of bisphenols; if used with epoxide resins which are more reactive towards them, such as epoxycycloaliphatic compounds, the storage stability of the mixtures is severely curtailed. A known drawback in using these hardeners with epoxide resins containing more than one 2,3-epoxypropyl group is the relatively lengthy time they require to be heated with such resins in order to bring about gelation and cure. It would clearly be desirable to have a mixture which cures very rapidly once it is heated above a certain critical temperature but which remains stable for prolonged periods below that temperature. We have found that this object can be achieved by inclusion in the epoxide resin-hardener mixture of certain organic oxidants.
Incorporation of an organic oxidant (benzoyl peroxide) into a composition containing an epoxide resin and a Lewis acid latent hardener (a boron trifluoride-triethanolamine complex) is known, having been described in Japanese Published Patent Application No. 103985/1974. The mixture described also contained an ethylenically unsaturated monomer, 2-hydroxyethyl acrylate, and the purpose of the oxidant was to effect polymerisation of this monomer.
Polymerisation of this monomer under conditions in which the epoxide resin remained uncured was demonstrated in unpublished experiments by the present applicant. A mixture of 19.1 parts by weight of an epoxide resin (a polyglycidyl ether of 2,2-bis(4-hydroxyphenyl)propane having a 1,2-epoxide content of 5.16 equivalents per kilogram), 0.86 part by weight of boron trifluoride-monoethylamine complex, and 2 parts by weight of 2-hydroxyethyl acrylate was stored at 25.degree. C. for 11 weeks, during which time its viscosity, measured at 25.degree. C., increased from 0.8 Pa s to 2.35 Pa s, i.e., by a factor of about 3. A similar mixture, but also containing 0.6 part by weight of a 50% dispersion of benzoyl peroxide in dibutyl phthalate, increased in viscosity over the same period and under the same conditions from 0.7 Pa s to 8.5 Pa s, i.e., by a factor of about 12. A similar mixture, but containing no 2-hydroxyethyl acrylate, increased in viscosity from 11 Pa s to 51 Pa s, an increase by a factor of 4.6. The large increase in viscosity that occurred only when both the monomeric acrylate and the peroxide were present is indicative of polymerisation of this monomer occurring on storage at 25.degree. C.
Incorporation of an epoxide resin with another polymerisable species, together with a curing agent for the epoxide resin and a peroxide curing agent for the other polymerisable species, is comparatively common. The following references illustrate this.
In U.S. Pat. Nos. 3,072,606 and 3,072,607 there are described compositions containing an epoxide resin, boron trifluoride, optionally as a complex with an amine, a peroxide and, in the first of these patents, a cyclic ether or thioether and, in the second, a compound containing at least two furan or thiophene rings. In both patents it is stated that a peroxide may be added as a conventional accelerator for copolymerisation of the epoxide resin with the cyclic compound. In the two examples in which a peroxide is used, the boron trifluoride is not present as an amine complex, cure of the mixture being effected rapidly at room temperature.
Compositions containing an epoxide resin, an unsaturated polyester, a boron trifluoride-amine complex, and an organic peroxide are described in West German Auslegeschrift No. 1 240 282 (Derwent C.P.I. Abstract No. 83839P). Peroxides are well-known catalysts for the cure of unsaturated polyesters, and their presence in such compositions is by no means unusual. There is therefore no indication that the peroxide has any effect on the cure of the epoxide resin.
Compositions containing a diallyl phthalate or dimethallyl phthalate prepolymer or telomer, an epoxide resin, an organic peroxide as polymerisation catalyst for the prepolymer or telomer, and a heat-curing agent for the epoxide resin, such as a boron trifluoride-amine complex, are disclosed in British Pat. No. 1,223,338. Use of a boron trifluoride-amine complex in such mixtures is not exemplified, and the Patent gives no information as to the effect of adding a peroxide to such a composition in the absence of the phthalate prepolymer or telomer.
Coating compositions containing polyvinyl chloride, an epoxide resin, an ester having at least three acrylic or methacrylic residues, and optionally also a peroxide and a hardener for the epoxide resin, such as a boron trifluoride complex, are disclosed in U.S. Pat. No. 3,657,381. Use of a boron trifluoride-amine complex is exemplified in a composition containing cumyl hydroperoxide, an epoxide resin, and a tetramethacrylate. The effect of mixing an epoxide resin with a latent curing agent and a peroxide in the absence of a free radical-curable monomer is not discussed.
Japanese Kokai No. 36800/1974 (Derwent C.P.I. No. 84988V/49), discloses the production of prepregs and laminates using a composition containing an epoxide resin, a bismaleimide, and a diamine. In an example such a composition also contained boron trifluoride-ethylamine complex and ethyl methyl ketone peroxide.
Finally, Japanese Kokai No. 58172/1975 (Derwent C.P.I. No. 85296W/52), describes the preparation of prepregs from compositions containing an epoxide resin (triglycidyl isocyanurate), a liquid polybutadiene, an organic peroxide and, optionally, a boron trifluoride-amine complex.
It is believed that incorporation of an organic oxidant into a mixture containing an epoxide resin and a boron trifluoride-amine complex, which mixture is substantially free from other cationically-polymerisable materials, is novel, and that the accelerating effect of the oxidant on the curing action of the boron trifluoride complex could not have been predicted from the disclosures of the above-mentioned prior art.
The acceleration by peroxides of the cure of epoxide resins with antimony compounds is described in British Patent Specification No. 1,033,263. The antimony compounds described in that Specification are not latent curing agents since they start to cure the epoxide resin at room temperature. The peroxide accelerates this room temperature cure. It is stated to be possible, using a less reactive antimony compound, a more stable peroxide, and a less reactive epoxide, to obtain mixtures that are stable at room temperatures for several days or weeks but which will polymerise rapidly on heating. Such extended stability is, apparently, brought about solely by the use of less reactive agents. The Specification does, however, show that peroxides are active accelerators for the cure of epoxide resins at room temperature. It is therefore surprising that a mixture of an epoxide resin, a latent hardener, and an organic oxidant is substantially as stable at room temperature as is the same mixture containing no organic oxidant and yet, when such mixtures are heated above their critical temperature, the mixture containing the oxidant cures at a much faster rate than does that with no oxidant.